A conventionally produced filter cigarette generally consists of a cylindrical column of tobacco, which is wrapped with a cigarette paper, and a filter, which is made from a filter material and is wrapped with a filter wrapper paper. A common filter material is cellulose acetate. Typically, the tobacco column and the filter are connected to each other by a tipping paper.
The residue remaining after the consumption of a filter cigarette consists in large part of the filter. This residue is in many cases not disposed of in an orderly manner, but is but simply thrown away, for which reason it remains in the environment until it disintegrates due to environmental influences. During the decomposition process, firstly the tipping paper and the filter wrapper paper detach from the filter material. This process happens relatively quickly, while, depending on the environmental conditions, the cellulose acetate fibers take between one month and three years to be disintegrated. Consequently, there is an interest in the industry for finding materials for cigarette filters that are degraded in the environment substantially faster than cellulose acetate fibers.
As an alternative to cellulose acetate fibers, paper is also known for use as a filter material for cigarettes. While paper generally degrades in the environment faster than cellulose acetate, the degradation of known paper filters still occurs more slowly than desired.
The speed of decomposition of paper in water can be measured with an apparatus that is described in TAPPI Method T 261 “Fines fraction by weight of paper stock by wet screening”. This apparatus consists of a cylindrical container with an internal diameter of 10 cm filled with warm distilled water at 23° C., the lower end of which is provided with a screen and closed by a discharge valve below the screen. In the container there is a stirring unit the speed of which can be set from 10 to 3000 revolutions per minute. The screen has 32 mesh per 25 mm and an opening width of 0.57 mm. Specifications regarding the stirring unit and its position in the container as well as further details of the apparatus can be taken from TAPPI T 261. The paper sample is put into the container while the stirring unit is running and the water with the paper sample is stirred for a defined period at a defined revolution speed. Then the water is discharged by opening the discharge valve, so that the fibers remain on the screen. The screen with the fibers is then dried and the fraction of disintegrated paper is determined by image analysis.
In detail, the measurement proceeds as follows. The paper to be measured is conditioned for at least two hours under the conditions defined in ISO 187. A small 20±0.5×20±0.5 mm sheet is cut out. At the start of the measurement, the container is filled with 800 ml of water. Then the stirring unit is turned on and set to a revolution speed of 800 revolutions per minute. The paper sample is put into the container, where it can disintegrate by the action of the shear forces caused by the stirring unit. The stirring unit is stopped 30 seconds after adding the sample and the water is immediately discharged by opening the discharge valve. Individual fibers and paper pieces, which did not disintegrate, remain on the screen.
After discharging the water the screen together with the fibers is dried in a drying oven for 5 minutes at 105° C. The amount of fibers left on the screen is determined by image analysis. To this end, the screen with the residues of the paper sample is placed on a black substrate and an image in grayscale with sufficient resolution is taken with a digital camera. This image is analyzed with appropriate software, for example the program “Image J”.
In the acquired digital image, the screen and individual fibers will appear dark, while non-disintegrated fiber bundles and larger residues of the paper appear light. A grayscale value is defined as the threshold that clearly distinguishes the screen and individual fibers from fiber bundles and paper residues. For a steadily increasing grayscale of 256 values characterized by values from 0 (black) to 255 (white), a value of 140 is adequate in many cases, whereupon, for a reasonable choice of threshold value, the result depends only to a small extent on the precise numerical value.
Afterwards, the number of pixels that have a grayscale value greater than the threshold value and hence belong to fiber bundles or larger residues of paper is counted. The ratio of the number of these pixels to the total number of pixels that corresponds to an undamaged paper sample of 20×20 mm is determined. This ratio is subtracted from 1 and expressed as a percentage. The higher the percentage, the more the paper has disintegrated.
In rare cases it can happen that the slightly disintegrated paper samples are deposited on the screen folded rather than flat. Because of the smaller visible paper surface, these samples would wrongly indicate greater disintegration of the paper. In such a case, the sample has to be discarded and the measurement has to be repeated with a new sample.
A result of at least 60% in this disintegration test corresponds to a paper that completely disintegrates in a container with water under slight movement within a few minutes, while conventional papers, for which the disintegration test delivers lower results, do not show any signs of disintegration under these conditions even after hours.
Cigarette filters manufactured from such conventional papers suffer from the disadvantage of degrading in the environment more slowly than desired. In general, it has to be said that conventional papers, which have a sufficient stability in the dry state, for example so that they can be processed by a machine, as a rule dissolve in water more slowly than desired for the purposes of the present invention.
Attempts have been made in the prior art to develop paper materials that dissolve in water comparatively quickly. An example in this regard is the use of unbleached pulp, from which indeed quickly disintegrating filters can be produced, but which, however, provides a filter with a light brown color, which is generally undesirable.